Be generous with the mount so it won’t cramp the image. Even a small (10cm) image looks good in a 30 cm frame – try it!
Cut the mount to leave some paper around the image itself – about 5% of its size on each side.
Don’t cut the mount to centre the image in the frame. (Framers should know this).
An optical illusion makes it look too low to the eye if it’s plumb centre.
So move it upwards (off centre) at least 1 cm and up to 3 cm or more in big frames.
A bevelled mount looks classy and helps frame the image.